Re: [Xen-devel] [PATCH 2/2] grant_table: convert grant table rwlock to percpu rwlock

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 17/11/15 17:39, Jan Beulich wrote:
>>>> On 17.11.15 at 18:30, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 17/11/15 17:04, Jan Beulich wrote:
>>>>>> On 03.11.15 at 18:58, <malcolm.crossley@xxxxxxxxxx> wrote:
>>>> --- a/xen/common/grant_table.c
>>>> +++ b/xen/common/grant_table.c
>>>> @@ -178,6 +178,10 @@ struct active_grant_entry {
>>>>  #define _active_entry(t, e) \
>>>>      ((t)->active[(e)/ACGNT_PER_PAGE][(e)%ACGNT_PER_PAGE])
>>>>  
>>>> +bool_t grant_rwlock_barrier;
>>>> +
>>>> +DEFINE_PER_CPU(rwlock_t *, grant_rwlock);
>>> Shouldn't these be per grant table? And wouldn't doing so eliminate
>>> the main limitation of the per-CPU rwlocks?
>> The grant rwlock is per grant table.
> That's understood, but I don't see why the above items aren't, too.

Ah - because there is never any circumstance where two grant tables are
locked on the same pcpu.

Nor is there any such need.

>
>> The entire point of this series is to reduce the cmpxchg storm which
>> happens when many pcpus attempt to grap the same domains grant read lock.
>>
>> As identified in the commit message, reducing the cmpxchg pressure on
>> the cache coherency fabric increases intra-vm network through from
>> 10Gbps to 50Gbps when running iperf between two 16-vcpu guests.
>>
>> Or in other words, 80% of cpu time is wasted with waiting on an atomic
>> read/modify/write operation against a remote hot cache line.
> All of this is pretty nice, but again unrelated to the question I
> raised.
>
> The whole interface would likely become quite a bit easier to use
> if there was a percpu_rwlock_t comprising all three elements (the
> per-CPU item obviously would need to become a per-CPU pointer,
> with allocation of per-CPU data needing introduction).

Runtime per-CPU data allocation is incompatible with our current scheme
(which relies on the linker to do some of the heavy lifting).

~Andrew

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